1. Field of Invention
This invention relates to the field of growth medium for bacteria and more specifically, this invention relates to selective plating media for the detection of specific bacteria strains or species. In particular, the present invention pertains to Salmonella or Shigella preferential selective plating media and use thereof.
2. Description of Prior Art
There exist a wide variety of commercially available plating media on the market, suggested as being preferential for Salmonella spp. Some of the more common selective plating agars are: MAC (Mac Conkey), HE (Hektoen Enteric); SS (Salmonella-Shigella); and XLD (xylose-lysine-deoxycholate). However, these well known agars, while capable of supporting the growth of Salmonella spp. and Shigella spp., are not as sensitive or as specific as one may need. This is because these agars do not adequately allow the differentiation of commonly encountered background bacterial growth (e.g. Citrobacter, and Proteus spp.) from the Salmonella and Shigella species which are to be detected.
The two present Applicants have contributed to the development of two different and more selective agars for detection of Salmonella spp. and Shigella spp. The first was XLT4, which comprised TERGITOL®4 (7-ethyl-2-methyl-4-undecanol hydrogen sulfate) in a conventional xylose-Lysine agar base medium, and additional promoters of H2S production, and is disclosed in U.S. Pat. No. 5,208,150, and incorporated by reference into the specification herein, in its entirety. This media is highly preferentially for Salmonella. The second selective agar was named MM (Miller-Mallinson) agar, which is disclosed in U.S. Pat. No. 5,871,944 and is also incorporated by reference into the specification herein, in its entirety. MM media was designed to detect strains of Salmonella that produce significantly lower amounts of H2S than other strains such as Salmonella. typhi and Salmonella Choleraesuis var. Kunzendorf. 
Applicants discovered, however, during the subsequent use of XLT4 and MM media, that while these selective growth agars could be successfully used for the great reduction of Proteus spp. growth, and in better differentiation of Citrobacter colonies in veterinary and food safety applications, they were not entirely successful for detecting Salmonella spp. and Shigella spp, in human diagnostic applications. XLT4 agar was found to be too inhibitory for Shigella spp. Moreover, while MM agar was found to be good at supporting growth and differentiation of Shigella flexneri, it did not differentiate Shigella sonnei colonies from colonies of E. coli and other lactose fermenting bacteria.
In general, the Salmonella/Shigella selective plating media currently available do not support the most efficient detection of salmonella species. MM and SS agar do not have the ability to differentiate Shigella sonnei from lactose-fermenting background colonies and reduces the sensitivity of these growth media for use in detection of an important human pathogen.
Therefore, there still exists a need for a selective growth plating media that provide sufficiently greater selectivity for Salmonella and Shigella colonies than current selective growth plating media, so that the selective growth media are capable of being used in human diagnostic applications and in food, feed and water quality control.